The chain on a chainsaw must stay sharp to properly pull the saw through the cut. Pushing or forcing the chain down is a signal that it's time to sharpen the chain. A small, round hand file should be used to shave off the dull edges. The size of the file must match the size of the chain to properly grind down the metal.
On most chains, a string of numbers is located on the side of the chain. These numbers signify the different measurements of the chain. For sharpening, the most important measurement is the chain's pitch. The pitch measures the distance between three consecutive rivets -- the tie straps holding the links together -- divided by 2. The smallest pitch is 6 mm (1/4 inch), and the most common is 1 cm (3/8 inch). Two other common sizes are 8.25 mm (0.325 inch) and 10.25 mm (0.404 inch). The pitch will determine the size of the file used for sharpening.
The file used for chainsaw sharpening is round. These files are specially cut to fit precisely into the hook or arch of the chain. This hook has three unique angles; all need to be sharpened precisely for the chain to cut. For this reason, only use round files intended for chainsaw sharpening. Chains with a pitch of 6 mm (1/4 inch and 9.5 mm (3/8 inch) generally take a file that's 5/32 inch, or 4mm. Chains with a pitch of a .325 and .404 will take a file that's 4.76 mm or 5.56 mm (3/16 or 7/32 inch), respectively. However, these file sizes aren't industry standards, so look on the package the chain came in, as it will state clearly the necessary file size.
Specially designed chains need a specially cut file to properly sharpen all the chain's angles. For most standard homeowner chains, a round file will adequately sharpen all three angles the teeth form. Some chains, mostly professional and commercial chains, are square ground, which means instead of a round hook, they have a flat, square-tooth point. These chains can only be sharpened with a tapered hand file. For the depth gauges in front of every tooth, a different file will be necessary.
The metal nubs in front of the cutting teeth are called the depth gauges. These gauges measure out how deeply the tooth will scoop into the wood. As the chain passes through the wood, the depth gauges score the wood; on the second pass, the tooth scoops out the wood. For the teeth to work properly, the depth gauges need to be flat-filed down as the teeth get smaller and smaller. Use a crosscut flat file, and move it parallel with the ground to file down the gauges.